An optical semiconductor memory in which light emission is performed with a small amount of trigger light to be continued even after the extinction of the trigger light is a basic device indispensable with an optical apparatus such as an optical switching, an information parallel processing system etc.
Such an optical semiconductor memory is described as an optical pnpn thyristor on pages 596 to 600 of "Journal of Applied Physics, 59 (2), 15 Jan. 1986". The conventional optical pnpn thyristor comprises an anode layer of p-AlGaAs, a cathode layer of n-AlGaAs, and a gate layer sandwiched between the anode and cathode layers wherein the gate layer is of a forbidden band narrower than those of the anode and cathode layers and includes an n-gate layer of n-GaAs and a p-gate layer of p-GaAs.
In operation, the pnpn thyristor is turned on to be shifted from a high impedance state to a low impedance state wherein carriers are mainly injected into the n-gate layer to be confined therein thereby emitting light with a high light emission efficiency. If a predetermined bias voltage less than a turning-on voltage is applied across the anode and cathode layers, light emission is continued even after the extinction of a trigger light so that the pnpn thyristor can be used for an optical memory.
In the conventional optical pnpn thyristor, however, there is a disadvantage that a considerable amount of electric power is consumed in maintaining the ON state by applying the predetermined voltage thereto under the extinction of the trigger light. According to the report of the aforementioned "Journal of Applied Physics", output light of 30 .mu.W is obtained with an injected current of 100 mA when voltage of approximately 1.5 V is applied across the anode and cathode layers. Therefore, a power consumption of 150 mW is resulted from one element of the pnpn thyristor. As compared to an electronic semiconductor memory such as S-RAM, D-RAM etc. the value of 150 mW is much higher so that it is difficult to put the pnpn thyristor into a practical use due to a construction that pnpn thyristors of the number as much as required must be arranged on a wafer.